Cell Division Biology Mr. Greene Unit 7

Bellringer Each time a cell reproduces, it divides into two new cells. When each of the new cells divide, the result is four new cells. If this continues, how many cells will be present after the cells reproduce 6 times?

Key Ideas Why do cells divide? How is DNA packaged into the nucleus? How do cells prepare for division?

Why Cells Reproduce As the body of a multicellular organism grows larger, its cells do not also grow large. Instead, the body grows by producing more cells. New cells are needed to help tissues and organs grow. As old cells die and new cells take their place. New cells also replace damaged cells. webbed fingers

Why Cells Reproduce, continued Cell Size A cell grows larger by building more cell products. To do this, the cell must take in more nutrients, process them, and get rid of wastes. A cell’s ability to exchange substances is limited by its surface area–to-volume ratio. As a cell gets larger, substances must travel farther to reach where they are needed.

Why Cells Reproduce, continued Cell Maintenance The work of cells is done by proteins. As a cell gets larger, more proteins are required to maintain its function. If the cell gets too large, DNA instructions cannot be copied quickly enough to make the proteins that the cell needs to support itself. Cell size is also limited by the cell’s DNA.

How Small Is a Cell? it varies the egg cell is 14 times as large as a red blood cell the contents of cells cannot be too far from the membrane. i.e. responses would take too long if cells were too big the smaller the cell, the more efficient it becomes i.e. look at the surface area to volume ratio

Ratio of Surface Area to Volume in Cells Cell Size Surface Area (l x w x 6) Volume (l x w x h) Ratio of Surface Area to Volume

Why Cells Reproduce, continued Making New Cells Each “daughter” cell has a higher surface area–to-volume ratio than its parent does. Each new cell also gets an entire copy of the cell’s DNA. Because larger cells are more difficult to maintain, cells divide when they grow to a certain size.

Chromosomes The large molecule of DNA is organized into hereditary units called genes. A gene is a segment of DNA that codes for RNA and protein. Each cell has a large amount of DNA that must be condensed into a very small volume. DNA is organized and packaged into structures called chromosomes.

Chromosome Structure it is a tightly coiled mass around proteins not visible because it is spread throughout the nucleus     chromosomes - tightly compacted bodies of DNA which arise before cell division (mitosis) chromatids - copies of each chromosome which form just prior to division centromere - protein disk which hold the chromatids together

Chromosome Number a set of chromosomes is called a karyotype there are 46 chromosomes in a single human cell you can sort them out into 23 pairs by size and shape homologous pairs = similar in shape and size and information. The pairs are almost identical copies of one another diploid (2n) - two copies of chromosomes haploid (n) - one copy of chromosomes Gamete cells (sperm and egg) have 1 homologue. Chromosome # in Organisms mosquito            6               fly                         12 frog                     26               mouse                 40 human               46               chimpanzee      48 orangutan         48               gorilla                  48 horse                   64               dog                     78 duck                          80

Sex Chromosomes sex chromosomes the pair which determines if you are a male or female Female = XX                     Male     = XY If you are male, you received your Y from your father. The sex chromosomes are separated during meiosis.

Visual Concept: Chromosomes

Chromosomes, continued A prokaryotic cell has a single circular molecule of DNA. This loop of DNA contains thousands of genes. A prokaryotic chromosome is condensed through repeated twisting or winding, like a rubber band twisted upon itself many times. parent cell DNA duplicates cell begins to divide daughter cells

Chromosomes, continued chromatid telomere centromere Eukaryotic Chromosomes Eukaryotic cells contain many more genes arranged on several linear DNA molecules. Eukaryotic DNA into highly condensed chromosome structures with the help of many proteins. The DNA and proteins make up a substance called chromatin. Condensed, duplicated chromosome

Chromosomes, continued Eukaryotic Chromosomes The first level of packaging is done by a class of proteins called histones. A group of eight histones come together to form a disc-shaped histone core. The long DNA molecule is wound around a series of histone cores in a regular manner and is called a nucleosome. Under an electron microscope, this level of packaging resembles beads on a string. The string of nucleosomes line up in a spiral to form a cord that is 30 nm in diameter. DNA double helix DNA and histones Chromatin Supercoiled DNA

Chromosomes, continued Eukaryotic Chromosomes During most of a cell’s life, its chromosomes exist as coiled or uncoiled nucleosomes. As the cell prepares to divide, the chromosomes condense even further ensuring that the extremely long DNA molecules do not get tangled up during cell division.

Chromosomes, continued Eukaryotic Chromosomes The nucleosome cord forms loops that are attached to a protein scaffold. These looped domains then coil into the final, most highly condensed form of the chromosome. Many dense loops of chromatin form the rod-shaped structures that can be seen in regular light microscopes. Each of the two thick strands of a fully condensed, duplicated chromosome are called a chromatid. Each chromatid is made of a single, long molecule of DNA.

Chromosomes, continued Eukaryotic Chromosomes Identical pairs, called sister chromatids, are held together at a region called the centromere. During cell division, the sister chromatids are separated at the centromere, and one ends up in each daughter cell. Each new cell has the same genetic information as the parent cell.

Preparing for Cell Division All new cells are produced by the division of preexisting cells. The process of cell division involves more than cutting a cell into two pieces. Each new cell must have all of the equipment needed to stay alive. All newly-formed cells require DNA, so before a cell divides, a copy of DNA is made for each daughter cell. Each new cells will function in the same way as the cells that they replace.

Preparing for Cell Division, continued Prokaryotes In prokaryotic cells, the circular DNA molecule is attached to the inner cell membrane. The cytoplasm is divided when a new cell membrane forms between the two DNA copies. Meanwhile the cell continues to grow until it nearly doubles in size. The cell is constricted in the middle, like a long balloon being squeezed near the center. Eventually the dividing prokaryote is pinched into two independent daughter cells, each of which has its own circular DNA molecule.

Binary Fission

Preparing for Cell Division, continued Eukaryotes The reproduction eukaryotic cells is more complex than that of prokaryotic cells. Eukaryotic cells have many organelles. In order to form two living cells, each daughter cell must contain enough of each organelle to carry out its functions. The DNA within the nucleus must also be copied, sorted, and separated.

Visual Concept: Comparing Cell Division in Prokaryotes and Eukaryotes Click above to play the video.

Summary Because larger cells are more difficult to maintain, cells divide when they grow to a certain size. Many proteins help package eukaryotic DNA into highly condensed chromosome structures. All newly-formed cells require DNA, so before a cell divides, a copy of its DNA is made for each daughter cell.

Bellringer Using the diagram of a football field, write a short paragraph describing how the players, the midfield line, and the goal posts compare to the structures of a cell involved in mitosis and cell division.

Key Ideas What are the phases of the eukaryotic cell cycle? What are the four stages of mitosis? How does cytokinesis occur?

Cell Division karyokinesis - the division of nuclear material mitosis - the process by which the nucleus of a eukaryotic cell divides to form two nuclei

Concept Map Cell Cycle includes M phase (Mitosis) Interphase is divided into is divided into G1 phase S phase Prophase G2 phase Metaphase Telophase Anaphase

Eukaryotic Cell Cycle The cell cycle is a repeating sequence of cellular growth and division during the life of a cell. The cell cycle is made up of five phases. The first three phases together are known as interphase. The remaining two phases make up cell division.

The Cell Cycle

Interphase G2 - second gap nerve cells are arrested in the G1 phase The longest part of a cells life                Three subphases G1 - first gap cell undergoes intense growth longest phase for most cells synthesize proteins                  S phase - synthesis copies of DNA are made 4 copies of each chromosome G2 - second gap usually the shortest of the 3 phases preparations for cell division duplicates its organelles nerve cells are arrested in the G1 phase referred to as G0 fibroblasts (heal wounds) grow on demand, otherwise quiescent

Visual Concept: Cell Cycle—G1 Phase Click above to play the video.

Visual Concept: Cell Cycle—S Phase Click above to play the video.

Visual Concept: Cell Cycle—G2 Phase Click above to play the video.

Visual Concept: Cell Cycle—M Phase Click above to play the video.

Visual Concept: Mitosis Click above to play the video

Mitosis – Stage 1 - Prophase first and longest phase of mitosis chromosomes coil into short rods = chromatids (sister) nuclear envelope breaks up centrioles form two tiny structures located in the cytoplasm near the nuclear envelope that separate and take up positions on opposite sides of the nucleus. spindle fibers - protein cables - form across the cell and enter the nucleus

Prophase Click to animate the image. C B F E A D

Mitosis – Stage 2 - Metaphase often only lasts a few minutes chromosomes attach to the spindle fibers at the kinetochores chromosomes line up in the center of the cell equatorial plane

Metaphase

Mitosis – Stage 3 - Anaphase shortest phase driven by microtubules each chromosome separates from its identical copy chromosomes are reeled to opposite sides of the cell spindle fibers begin to break down

Anaphase

Mitosis – Stage 4 - Telophase each side now has a complete set of chromosomes nuclear envelope now develops around each new set of chromosomes chromosomes uncoil spindle fibers disappear

Telophase

Visual Concept: Snapshot of Mitotic Structures

Finishing Up the Division CYTOKINESIS Mitosis is followed by cytokinesis - cell division the cell divides into two cells with its own nucleus Animal Cells the cell membrane is drawn inward until the cytoplasm is pinched in half (cleavage furrow) forming two new cells (daughter cells) Plant Cells plant cells develop a cell plate midway between the 2 new ones the cell wall then begins to appear in the cell plate     sometimes karyokinesis occurs without cytokinesis and multinucleated cells result

The Cell Cycle G1 phase M phase S phase G2 phase

Visual Concept: Comparing Cell Division in Plants and Animals

Summary The life of a eukaryotic cell cycles through phases of growth, DNA replication, preparation for cell division, and division of the nucleus and cytoplasm. Mitosis is a continuous process that can be observed in four stages: prophase, metaphase, anaphase, and telophase. During cytokinesis, the cell membrane grows into the center of the cell and divides it into two daughter cells of equal size. Each daughter cell has about half of the parent’s cytoplasm and organelles.

Bellringer You have learned how cells divide. One cell divides into two daughter cells. Each of those daughter cells divide into two more daughter cells. Make a graph that shows how the number of cells increases.

Key Ideas What are some factors that control cell growth and division? How do feedback signals affect the cell cycle? How does cancer relate to the cell cycle?

Controls Cell division is highly controlled. Cell growth and division depend on protein signals and other environmental signals. Many proteins within the cell control the phases of the cell cycle. Signals from surrounding cells or even from other organs can also regulate cell growth and division. Environmental conditions, including the availability of nutrients, also affect the cell cycle.

Visual Concept: Control of the Cell Cycle

Checkpoints During the cell cycle, a cell undergoes an inspection process to ensure that the cell is ready for the next phase in the cell cycle. Feedback signals at key checkpoints in the cell cycle can delay or trigger the next phase of the cell cycle. There are three main checkpoints in the cell cycle—G1 Checkpoint, G2 checkpoint, mitosis checkpoint.

Checkpoints

Checkpoints, continued G1 Checkpoint Before the cell copies its DNA, the cell checks its surroundings. If conditions are favorable and the cell is healthy and large enough, the cell enters the synthesis phase. If conditions are not favorable, the cell goes into a resting period. Certain cells, such as some nerve and muscle cells, remain in this resting period for a long time. They do not divide very often.

Checkpoints, continued G2 Checkpoint Before mitosis begins, the cell checks for any mistakes in the copied DNA. Enzymes correct any mistakes. This checkpoint ensures that the DNA of the daughter cells will be identical to the DNA of the original cell. Proteins also double-check that the cell is large enough to divide. If the cell passes the G2 checkpoint, then the cell may begin to divide. Once past this checkpoint, proteins help to trigger mitosis.

Checkpoints, continued Mitosis Checkpoint During the metaphase stage of mitosis, chromosomes line up at the equator. At this point, the cell checks that the chromosomes are properly attached to the spindle fibers. Without this point, the sister chromatids of one or more chromosomes may not separate properly. This checkpoint ensures that the genetic material is distributed equally between the daughter cells.

Cancer Each year, more than 1 million Americans are diagnosed with cancer. Cancer is a group of severe and sometimes fatal diseases that are caused by uncontrolled cell growth. Uncontrolled cell growth and division can result in masses of cells that invade and destroy healthy tissues. Preventing or curing cancer requires an understanding of how a healthy person’s cells can become cancerous.

Cancer, continued Loss of Control Normally, a cell responds properly to signals and controls. However, damage to a cell’s DNA can cause the cell to respond improperly or to stop responding leaving the cell cycle uncontrolled. The defective cell divides and produces more defective cells. Eventually, these cells can form a mass called a tumor.

Cancer, continued Development A benign tumor does not spread to other parts of the body and can often be removed by surgery. A malignant tumor invades and destroys nearby healthy tissues and organs. Malignant tumors, or cancers, can break loose from their tissue of origin and grow throughout the body. This process is called metastasis. Once a cancer has metastasized, it becomes difficult to treat.

Cancer, continued Treatment Some cancers can be treated by using drugs that kill the fast-growing cancer cells. Because drugs are chemicals, this method of treatment is called chemotherapy, or “chemo” for short. Some cancers can be treated by surgery to remove of the affected organ. In radiation therapy, high-energy rays are focused on an area in order to destroy cancerous cells.

Cancer, continued Prevention The best way to prevent cancer is to avoid things that can cause cancer. Ultraviolet radiation in sunlight can damage genes that control the cell cycle. Chemicals in cigarette smoke also affect how cell growth and division is regulated.

Summary Cell growth and division depend on protein signals and other environmental signals. Feedback signals at key checkpoints in the cell cycle can delay or trigger the next phase of the cell cycle. Uncontrolled cell growth and division results in tumors, which can invade surrounding tissues and cause cancer.